This law, known as Gay-Lussac's law, observes the relationship between the pressure and temperature of a gas. Contrary to its name, this relationship was actually discovered by French scientific instrument inventor and physicist Guillaume Amontons, and is sometimes referred to as Amontons' pressure-temperature law. Although Guy-Lussac explored the temperature-pressure relationship, the Guy-Lussac law is generally used to refer to the law of volume combination. Amontons hesitated on this relationship while he was building an “air thermometer”. Although few people have been able to identify its exact method of experimentation, scientists later developed a device consisting of a pressure gauge and a metal sphere. These two pieces were then attached and immersed in solutions at different temperatures. From the research and experiments of Amontons and Guy-Lussac, they determined that pressure and volume had a direct relationship; as one increased, the other increased. It was then found that the quotient of pressure and temperature was equal to a constant in which, just like Boyle's law, one could use either variable at another pressure or temperature, given the one of the variables and the other conditions remain the same. . Instead of using various solutions at different temperatures as in the experiment described above, many experiments today use a solution in which the temperature is increased or decreased, as in the following example.